Steel strapping



L. S. SHELTON STEEL STRAPPING Jan. 14, 1969 Filed April 8, 1966 Sheet O INCHES PRESTRESS BY 90 REVERSE BEND ED RADIUS GmEiSEEw H 352: 1

0 NO BEND O NO BEND PRESTRESS BY 90 REVERSE BEND RADIUS-INCHES TWICE PRESTRESSED BY I80 REVERSE BEND 6) RAD-IN.

lNVE/VTOR omvz/zce 5 j fg 5 3% W W gm ATTORNEKS L. S. SHELTON STEEL STRAPPING Jan. 14, 1969 Sheet Filed April 8, 1966 .062 .O3l O REVERSE BEND E1) RADlUS-lNCHES O O O m m s NO BEND .093 PRESTRESS BY 90 O 0 m m O -i NO BEND .054

PRESTRESS BY 90 REVERSE BEND RADIUS-INCHES A T'TOR/VEYS Jan. 14, 1969 s, s L-ro 3,421,951

STEEL STRAPPING Filed April 8. 1966 Sheet 4 0 or 4 HIGH \TENSILE I 50 I00 I50 200 250 300 i 400 NO. FLEXURAL CYCLES TENS! LE STRENGTH- MP8! a 200 J x E P g .60 M z w A g I20 i g 80 S E 40 E E o APPLIED TENSION-i? W" INVENTOR z mfiezcce 5 5%e6i'0za 3 27% A ZLWJZ C'WfM A TTORNEYS United States Patent 6 Claims This invention relates to metallic ligatures. More particularly, it relates to steel strapping which combines a relatively high tensile strength and enhanced toughness with unusual ductility as measured by benda'bility.

In accordance with this invention, relatively thin steel strapping of a cross-sectional thickness in the range between about 0.005" and 0.065 is obtained having a tensile strength in the range between about 175,000 p.s.i. and 250,000 p.s.i. after being subjected to bending associated with its use in the tying of packages, etc. where prestressing to a permanent deformation before and during tensioning may be of the order of a 90 bend or greater depending upon the configuration of the objects being tied.

Steel strapping is a relatively thin steel ligature usually with cross-sectional thickness in the range between about 10 mils and of a width generally in the range between about /4 and 2". Methods are known by which steel having a high tensile strength of the order of 150,000 p.s.i. to 300,000 p.s.i. can be produced, but such steel is not suitable for use as strapping because it lacks the ductility, as measured by bendability necessary for conventional strapping operations.

It also work hardens rapidly so that a progressive deterioration of embrittlement occurs during strapping or similar operations. It is also possible to obtain ligatures with excellent ductility but such ligatures which have been available commercially heretofore exhibited low tensile strength which drastically limited the utility of these ductile ligatures.

Most of the steel strapping currently available evidences a compromise on properties whereby reasonable ductility and a tensile strength of the order of 90,000 p.s.i. to 160,000 p.s.i. are obtained.

Now it has been discovered that, contrary to accepted tenets of the steel trade that as tensile strength increases, toughness and ductility decrease, steel strapping with enhanced tensile strengths, i.e., as much as 75% higher than commercially available strapping of the same dimensions can be produced while at the same time attaining toughness and ductility markedly superior to those of conventional strapping.

As a consequence of the improvement developed in ductility, toughness and tensile strength, the load bearing ability of conventional straps can be obtained with thinner strapping, i.e., strapping of reduced cross-sectional area. This thinner strap requires less force to make bends, for example, around the corners of packages, and to seal straps, factors permitting substitution of a strap having a lower cost per foot of length than currently available strap. A further advantage of thinner straps is the greater ease with which the strap conforms to the geometry of the object being strapped without fracturing at the corners and without crushing the corners of the strapped object. Such conformity to the contour of the objects being strapped is important because strapping which fits snugly to contours, acts most efficiently to absorb shock and to prevent load shifting.

The strapping of this invention is produced when the strapping is so processed as to provide a product having identifiable core and surface layer portions, said core portion constituting between about 40% and 90% of the strap thickness and having a substantially uniform distribution of carbon throughout the core thickness in the range be- "ice tween about 0.2% and 0.85% and a crystalline structure characterized by the presence of martensite, martensite plus bainite or bainite and said surface layer portion, i.e., top and bottom, constituting between about 5% and about 30% of the cross-sectional thickness of the strap, said surface layers being identifiable by physical structure different from that of the core due to the lower carbon content and having an average carbon content at a depth in the range between about 0.0015 and 0.0025 inch from the outer surface such that the ratio of carbon content of the core to said carbon content at the specified depth is in the range between about 2:1 and 15:1 and having a carbon content in the zone intermediate said core and said specified portion of said outer surface layer which decreases progressively as the distance from the core increases.

In this instance, the language decreases progressively is not intended to infer a straight line relationship but merely that the carbon content of any area is less than that of the contiguous area nearer to the core.

The steel strapping of this invention may have a cross* sectional thickness of 0.005 to 0.065". In producing a steel strapping having these cross-sectional dimensions, a steel sheet generally having a carbon content in the range between 0.2% and 0.85% and a manganese content in the range between about 0.3% and about 1.4% is the starting material although other materials may be used. After production of a surface layer at each face of the strapping material which has a carbon content as hereinafter defined, the steel has a core area which contains a carbon content which is the same or different from that of the starting metal, but still within the range between about 0.2% and 0.85 preferably in the range between about 0.25% and 0.65%, uniformly distributed in a core preferably constituting between 50% and of the strap thickness. This steel material which can be considered to be a substantially three layer material, is then heat treated to produce tensile strengths in the range between 175,000 p.s.i. and 250,000 p.s.i., preferably at 180,000 p.s.i. and 220,000 p.s.i. under which conditions the structure in the core area is generally identifiable as containing martensite and/or bainite.

In such a predominantly three layer strapping, if the surface layers have been decarburized to the point where, upon reduction in cross section, each surface layer will still constitute between about 5% and about 30% of the total cross-sectional thickness, the steel material may be rolled to give a cross-sectional reduction in thickness, for example, of the order of between 1.5 :1 and about 10:1. Such rolled steel has the advantage that the thickness of the decarburized layers can be controlled with greater accuracy than can the thickness of surface layers produced by decarburizing operations on strap already reduced to final thickness. However, strap having the character described can be prepared by reducing sheet to strap thickness using conventional rolling of low carbon sheet under conditions designed to maintain the integrity of the sheet without the necessity of intermediate annealing as is present conventional practice with higher carbon steel and then effecting the necessary carburizing and decarburizing operations on the rolled sheet.

After such reduction in thickness of the steel, the thin sheet may then be subjected to the heat treatment to produce a core having a predetermined tensile strength compatible with the carbon content present.

The carbon content of the core area of the steel strapping is an important factor in the attainment of desired properties. If the starting steel has too high or too low a carbon content, the steel may be subjected to suitable decarburizing or carburizing treatments. For example, if the carbon content of the starting steel is 0.2% and it is desired to have a carbon content in the strapping core of 0.35%, the steel may be subjected to carburizing treatment as taught in U.S. Patent No. 3,109,877 to introduce the additional carbon required. This carburized steel may then be subjected to the decarburizing step required to produce surface layers of carbon content low enough so that the surfaces after the heat treatment will be essentially a ductile low carbide content steel.

A more complete understanding of the strap and how it differs in characteristics from other conventional strapping may be had by reference to the following description of tests and the graphs comparing the strap of this invention with other strapping products illustrated in the drawings in which:

FIGURE 1 is a graph with the ordinate showing tensile strength of various strapping materials of the same thickness and width, the commercially available straps being the type having a tensile strength in the range between 110,000 p.s.i. and 160,000 p.s.i. in the unstressed condition, after subjection to conditions of stress varying from zero to that condition created by bending the strap around a sharp corner of substantially zero radius, after which the strap is straightened prior to tensile strength determination;

FIGURE 2 is a graph with the ordinate showing toughness of the various strapping materials of the same thickness and width, the commercially available straps being the type having a tensile strength in the range between 110,000 p.s.i. and 160,000 p.s.i. in the unstressed condition, after subjection to the conditions of stress varying from zero to that condition created by bending the strap around a sharp corner of substantially zero radius, after which the strap is straightened prior to testing, the toughness being plotted in inch pounds per inch of lengths per inch square times 10 a measure of the area under the stress-strain curve during a tensile test with the strap pulled in elongation until fracture occurs;

FIGURE 3 is a graph with the ordinate showing tensile strength of various strapping materials of the same thickness and width, i.e., .025" thick and 0.75" width, the commercially available straps being the type having a tensile strength in the range between 110,000 p.s.i. in the unstressed condition, under twice applied conditions of stress created by bending the strap materials around objects having corners of radii varying from a 0.93 inch radius to a .031 inch radius, the testing known as prestressing by 180 reverse bend;

FIGURE 4 is a graph comparing tensile strengths of various strapping materials of the same thickness and width, i.e., .020 inch thick and 0.75 inch width, the strappings compared to the strap of this invention being the type having a high tensile strength in the range between 175,000 p.s.i. and 250,000 p.s.i., after subjection to conditions of stress varying from zero to that condition created by bending the strap around a sharp corner of substantially zero radius, after which the strap is straightened prior to tensile strength determination;

FIGURE 5 is a graph with the ordinate showing breaking load of various strapping materials of different thickness but same Width, the commercially available strap being the type of 0.035 inch thickness having a tensile strength in the range between 110,000 p.s.i. and 160,000 p.s.i. in the unstressed condition, after subjection to conditions of stress varying from zero to the condition created by bending the strap around objects having corners of radii varying from .054 inch radius to a .018 inch radius, after which the strap is straightened prior to tensioning compared to strap of this invention of 0.018 inch thickness;

FIGURE 6 is a graph with the ordinate showing breaking load of various strapping materials of the same width and dilferent thickness, one strap being of the type having a tensile strength in the range between 175,000 p.s.i. to 250,000 p.s.i. in the unstressed condition, after subjection to conditions of stress varying from zero to the condition created by bending the strap around objects having corners of radii varying from .093 inch to .031 inch radius, after which the strap is straightened prior to tensioning;

FIGURE 7 is a graph with the ordinate showing tensile strength of various strapping materials of the same thickness and width, the commercially available strap being the type having a tensile strength in the range between 110,000 p.s.i. and 160,000 p.s.i. in the unstressed condition and the abscissa showing the number of fiexural cycles of the strap before failure;

FIGURE 8 is a graph with the ordinate showing toughness in inch pounds per inch and the abscissa showing the number of fiexural cycles of the strap before failure;

FIGURE 9 is a graph with the ordinate showing tensile strength of various strapping materials of the same thickness and width, one strap being of the type having a tensile strength of 190,000 p.s.i. in the unstressed condition and strapping of this invention having a tensile strength of 230,000 p.s.i. and the abscissa showing the number of fiexural cycles of the strap before failure; and

FIGURE 10 is a graph with the ordinate showing tensile strength retained by various strapping materials and abscissa showing the applied tension after prestressing around a corner of almost zero radius, the commercial available straps used for comparative purposes being the type having a tensile strength before prestressing in the range between 110,000 and 160,000 p.s.i.

In the tests, the data from which is plotted in the graphs, a number of specimens of each of the steel straps were used so that an average result could be plotted. The letter P identifies strap of this invention. The letters A, .M and H indicate straps with properties which are discussed where relevant to an understanding of the data.

Reference to FIGURE 1, dealing with tensile strength versus radius of prestressing bend, shows that strap P has more tensile strength than strap A and 50% more tensile strength than strap M in the unstressed condition. On the other hand, after prestressing around corners of various radii, strap P of this invention has more tensile strength than strap A and 80% more tensile strength than strap M.

Reference to FIGURE 2 gives a comparison of the toughness of various straps having the same width and thickness.

Toughness is defined as the area under the stress-strain curve up to the point of rupture and is a measure of the ability of the strap to absorb energy in the plastic range. This property comes into play when steel strapping is subjected to impact or shock loads, as when used on carload loadings of lumber, bricks, etc, where, after the strap has been bent around a sharp corner of, for example, a brick pack, it must retain the ability to absorb impact loads and not snap when jolts occur during transportation.

Under conditions of no prestressing by bending, strap P of this invention exhibits greater toughness than strap A and about the same toughness as strap M. When prestressed by bending even at a gentle radius of 0.093", strap A has lost 60% and strap M has lost 25% of its original toughness whereas the toughness of strap P of this invention remains unchanged. When subjected to prestressing by bending around a sharp corner, i.e., corners with substantially O radius, both straps A and M have lost approximately 98% of the toughness, i.e., snap readily Whereas strap P of this invention has lost only 25% of its original toughness.

Reference to FIGURE 3, dealing with the effect on the tensile strength of strap of the same width and thickness of prestressing by bending twice to a permanent deformation around anvils of various radii simulating various types of bends encountered in strapping operations, before straightening for the tensile test, i.e., a test which is more severe than that for which data is given in FIGURE 1 and of a nature more comparable to what can occur in strapping operations, shows that when subjected to gentle bending around relatively large anvils, the straps, i.e., conventional straps A and M and strap P of this inVentiOn, suffer little loss in tensile strength. On the other hand, when the conventional straps A and M are bent around an anvil of radius of the order of .031", the conventional straps lose substantially all of the tensile strength whereas strap P suffers only 5% drop in tensile strength.

Reference to FIGURE 4, which compares the tensile strength of a so-called high tensile strength strap H and strap P of this invention having the same width and thickness before and after prestressing, shows that prior to prestressing by bending, both strap H and strap P have approximately the same tensile strength of 240,000 pounds per square inch. However, strap H, even after gentle presstressing by bending around anvils of relatively large radii shows the rapid decline in tensile strength evidencing a lack of ductility. Strap P, on the other hand, shows only a loss in tensile strength of about 12% even when bent around anvils of small radii approaching 0 radius. The decline in tensile strength of strap H is at least in part due to fracture of the strapping at the corners.

Reference to FIGURE 5, which prewnts data on breaking loads for strap, one of which is a conventional strap of inch width and 0.035 inch thickness of the same conventional type as strap A of FIGURE 1 and differing therefrom only in thickness and the other is strap P of this invention which has the same width as the conventional strap A but only 0.02 inch thickness, shows that prior to prestressing the straps A and P will break under substantially the same load. On the other hand, strap P when prestressed by bending shows only a gradual reduction in load carrying ability, with the reduction being only 12% after bending around an anvil or corner of 0.018 inch radius whereas strap A despite its greater thickness has lost 80% of its load-carrying ability after prestressing by bending around a corner of 0.036 inch radius.

Reference to FIGURE 6, which presents data on breaking loads for a high strength strap of 180,000 per square inch tensile strength, inch width and 0.025 inch thickness and for strap P of this invention of the same width but only 0.020 inch thickness, shows that prior to prestressing, straps P and H will break under substantially the same load.

Strap P, when prestressed by bending, shows substantially no reduction in the load that it will sustain in tension whereas strap H even when bent around a radius of 0.05 inch retains only 35% of its ability to sustain loads.

Reference to FIGURE 7, which compares the fatigue resistance of straps of the same width and thickness, shows the superiority of strap P of this invention. Fatigue resistance is determined by tensile strength of the straps after being subjected to various flexural cycles. This test was performed in a machine where one end of the strap is clamped in a special vice and held stationary while the other end of the strap is engaged by a vertically oscillating yoke. The strap is flexed to a permanent deformation through an arc of about 45 inclusive angle. After flexure for a number of cycles in increments of generally 50, the strap is subjected to a standard tensile test to determine the stress-strain curve. The fiexure test measures short cycle fatigue resistance or the ductility of the strap. After 50 cycles for the conventional strap A and 200 cycles for the conventional strap M, these straps had failed completely in tensile strength whereas strap P maintains its tensile strength until shortly before fracture at 513 cycles.

Reference to FIGURE 8, which gives a comparison of the toughness, as defined in connection with the discussion of FIGURE 2, of straps of the same width and thickness, shows the superiority of strap P of this invention.

The graph shows that the toughness of conventional straps fails rapidly. At 20 flexure cycles for strap A and 150 cycles for conventional type strap M, these straps had nearly failed in toughness whereas strap P had toughness reliability until shortly before fracture at 513 cycles.

Reference to FIGURE 9, which gives a comparison of the fatigue resistance of straps of the same width and thickness, shows the superiority of strap P of this invention over the high tensile strength strap H. Stra H, in this instance, had an initial tensile strength of 190,000 pounds per square inch and a fair degree of ductility and strap P had an initial tensile strength of 230,000 pounds per square inch. After cycles, the tensile strength of the strap H rapidly decayed and complete fracture occurs at 200 cycles whereas strap P maintained its tensile strength without appreciable diminution until it failed at 513 cycles.

Reference to FIGURE 10, which presents data on the retained tensile strength of straps of the same width and thickness, after bending of the strap to a permanent deformation around an anvil of 0.062 inch radius while subjected to the indicated tensions, shows that up to 1000 pounds tension, all of the straps show only slight reduction in tensile strength. When bent under tensions exceeding 1000 pounds, straps A and M show a rapid loss of tensile strength while strap P of this invention exhibits little loss in tensile strength until tension exceeding 2000 pounds is exerted. This retained tensile strength provides a wide margin of safety in strapping operations and eliminates the need to prepare straps tailored to very limited types of strapping operations.

Another superior characteristic of the straps of this invention is the strength of joints such as the notch type joint where overlapping portions of strap are secured together. Joints prepared in a conventional joint forming operation using an AHP air tool and subjected to tension in a standard tensile testing machine broke when strap A joints were subjected to 1320 pounds tension, when strap M joints were subjected to 1700 pounds tension and when strap P joints were subjected to 2500 pounds tension. Joints made from strap P thus are almost 100% stronger than joints of strap A and almost 50% stronger than joints of strap M.

According to this invention, a steel strapping having a carbon content of approximately 0.35%, which by conventional treatments can produce a steel strap capable of developing a tensile strength in the range between 130,000 and 200,000 p.s.i. or higher but subject to the disabilities of lessened ductility and increased brittleness as the strength is increased, may be processed to produce a product in accordance with this invention such as when the surface is decarburized and the ultimate strap product has a total thickness of 0.018 inch, the product may have surface layers of approximately 0.004 inch thickness and a carbon content of approximately 0.06% as an average for the area between about 0.0015 inch and 0.0025 inch from the surface. Such a product, if subjected to austenizing heat treatment can produce a strap having the ductility characteristics of the strap of this invention as well as having a tensile strength of approximately 200,000 p.s.i.

Control over the carbon content of the surface layers is important to attainment of the objectives of this invention. The method, time of treatment and composition of the decarburizing operation gases, will have marked effect upon the character of that portion of the surface layer immediately adjacent the external surface. For example, the treatment can impart a relatively uniform carbon content to the, for example, first 0.002 inch of surface layer adjacent the external surface or this portion of the surface can show some variation in carbon content but all decarburizing operations should be operated to establish at a depth between about 0.0015 inch and 0.0025 inch from the outer surface, a carbon con tent zone such that the ratio of carbon content of the core to said carbon content at the specified depth is in the range between about 2:1 and 15:1 and such that in Inch depth Percent carbon 0.0015 to 0.0025 0.10 0.0025 to 0.0035 0.18 0.0035 to 0.0045 0.28

This decrease in carbon content with increase in distance from the core is to be compared with the carbon content distribution in currently commercially available straps:

0.6% carbon, 0.42% carbon, 0.24% carbon, Width, width, Width, 0.035" thiek- 0.044 thick- 0.031" thickness, percent ness, percent ness, percent 0.0015-0.0025 0. 62 O. 42 0. 22 0.0025-0.0035- 0. 02 0.42 0.21 00035-00045. 0. 61 0.42 0.22 Core 0. 59 0. 42 0.24

A further improvement in properties of the strap of this invention can be produced if a spherodization cycle is included in the processing after decarburization. When the decarburization cycle is completed, the furnace is cooled fast to 1350 F. and then slow cooled to 1200 F. at the rate of 15 F. per hour.

The spherodization cycle produces in the steel a structure of globular spheroidal carbides in a ferrite matrix. This structure reduces the tendency for microcracks during rolling, facilitating cold working. In addition, spherodization prepares the steel for maximum heat treatability resulting in improved tensile strength, notch toughness and increased elongation.

While the invention has been described with reference to specific embodiments, it will be apparent that other adaptations will be recognized by those skilled in the art. The strap, for example, may be prepared from a variety of steel starting materials. A starting steel may be a hot rolled steel of various thicknesses or it may be an initially thick steel which has been cold rolled to an intermediate thickness or a strapping thickness. It will also be recognized that steel of less than optimum carbon content may be carburized to insure a uniform distribution of the optimum quantity of carbon which is desired in the strapping core. It will further be recognized that the tensile strength is not only dependent on core thickness and core carbon content but also upon heat treatment, quenching conditions, etc. Therefore, the details are intended to be merely illustrative and modifications may be made without departing from the spirit and scope of the invention as stressed in the appended claims.

I claim:

1. A steel strapping prepared from carbon steel having a carbon content in the range between 0.2% and 0.85% and a manganese content in the range between 0.3% and 1.4%, having a cross-sectional thickness in the range between about 0.010 inch and about 0.065 inch, said crosssectional thickness having core and surface layer portions, identifiable by differing crystalline structures, said core portion constituting between about 40% and 90% of the strapping thickness and having substantially uniform distribution of carbon throughout the core thickness in an amount in the range between about 0.2% and 0.85% and each of said surface layer portions constituting between about 5% and about 30% of the total strap thickness, said surface layers being identifiable by physical structure ditferent from that of the core due to the lower carbon content and having an average carbon content at a depth in the range between about 0.0015 inch and 0.0025 inch from the outer surface such that the ratio of carbon content of the core to said carbon content at the specified depth is in the range between about 2:1 and 15:1 and having a carbon content in the zone intermediate said core and said specified portion of said outer surface layer which decreases progressively as the distance from the core increases.

2. A steel strapping according to claim 1 wherein carbon content of the core is 0.25% to 0.65% uniformly d stributed throughout the thickness of the core.

3. A steel strapping according to claim 1 wherein the thickness of the strap is in the range between about 0.01 inch and about 0.025 inch.

4. A steel strapping according to claim 1 wherein the core portion constitutes between about and of the strapping thickness.

5. A steel strapping according to claim 1 wherein there is an intermediate zone between said surface layers and said core having a carbon content which is intermediate that of the identifiable core and surface portions and decreases progressively as the distance from the core increases.

6. A steel strapping as claimed in claim 1 having a cross-sectional thickness of 0.018 inch, said core portion having a thickness of 0.014 inch and having approximately 0.33% of carbon and approximately 1% of manganese uniformly distributed in said core, and each of said surface layer portions having a thickness of approximately 0.002 inch and having an average carbon content at a depth in the range between about 0.0015 inch and 0.0025 inch from the outer surface such that the ratio of carbon content of the core to said carbon content at the specified depth is in the range between about 2:1 and 15:1 and having a carbon content in the zone intermediate said core and said specified portion of said outer surface layer which decreases progressively as the distance from the core increases, said steel strapping having a tensile strength after bending about an anvil to a permanent deformation of degrees of approximately 200,- 000 p.s.i.

References Cited UNITED STATES PATENTS 2,455,331 11/1948 Eckel et a1. 148-121 X 2,832,115 4/1958 Schindler 24-20 3,152,020 10/1964 Gross 148-39 X 3,311,512 3/1967 Mohri et al. 148-12.] 3,323,953 6/1967 Lesney 148-39 OTHER REFERENCES Transactions of ASM, vol. 37, 1946, relied on; pp. 4850 and 56-68.

CHARLES N. LOVELL, Primary Examiner.

US. Cl. X.R. 148-121; 24-20 

1. A STEEL STRAPPING PREPARED FROM CARBON STEEL HAVING A CARBON CONTENT IN THE RANGE BETWEEN 0.2% AND 0.85% AND A MANGANESE CONTENT IN THE RANGE BETWEEN 0.3% AND 1.4%, HAVING A CROSS-SECTIONAL THICKNESS IN THE RANGE BETWEEN ABOUT 0.010 INCH AND ABOUT 0.065 INCH, SAID CROSSSECTIONAL THICKNESS HAVING CORE AND SURFACE LAYER PORTIONS, IDENTIFIABLE BY DIFFERING CRYSTALLINE STRUCTURES, SAID CORE PORTION CONSTITUTING BETWEEN ABOUT 40% AND 90% OF THE STRAPPING THICKNESS AND HAVING SUBSTANTIALLY UNIFORM DISTRIBUTION OF CARBON THROUGHOUT THE CORE THICKNESS IN AN AMOUNT IN THE RANGE BETWEEN ABOUT 0.2% AND 0.85% AND EACH OF SAID SURFACE LAYER PORTIONS CONSTITUTING BETWEEN ABOUT 5% AND ABOUT 30% OF THE TOTAL STRAP THICKNESS, SAID SURFACE LAYERS BEING IDENTIFIABLE BY PHYSICAL STRUCTURE DIFFERENT FROM THAT OF THE CORE DUE TO THE LOWER CARBON CONTENT AND HAVING AN AVERAGE CARBON CONTENT AT A DEPTH IN THE RANGE BETWEEN ABOUT 0.0015 INCH AND 0.0025 INCH FROM THE OUTER SURFACE SUCH THAT THE RATIO OF CARBON CONTENT OF THE CORE TO SAID CARBON CONTENT AT THE SPECIFIED DEPTH IS IN THE RANGE BETWEEN ABOUT 2:1 AND 15:1 AND HAVING A CARBON CONTENT IN THE ZONE INTERMEDIATE SAID CORE AND SAID SPECIFIED PORTION OF SAID OUTER SURFACE LAYER WHICH DECREASES PROGRESSIVELY AS THE DISTANCE FROM THE CORE INCREASES. 